Tanks for storing liquid include a so-called liquid tank for storing water or flammable oils such as gasoline and diesel fuel. Such tanks store liquid therein. The liquid is pumped to outside of the tank through piping or the like, if desired. At times, in the tank, the position of the gas-liquid interface (namely, liquid level) lowers. The volume of a gas space vertically above the liquid level in the tank increases with lowering in the liquid level. In the case that the space in the tank is closed, the pressure in the gas space decreases as the liquid level moves vertically downwardly. When the pressure in the gas space reaches a value close to the vacuum, it becomes difficult to pump the liquid from the tank to outside of the tank.
For this reason, a tank for liquid generally has a vent pipe to provide communication between the above-mentioned gas space and the space outside the tank (namely, atmosphere). By introducing the air outside the tank into the gas space through the vent pipe, it is possible to bring the pressure in the gas space close to ambient atmospheric pressure. By suppressing the reduction of pressure in the gas space, it becomes easy to pump the liquid below the gas space to outside of the tank.
Such vent pipes include a so-called “non-valve vent pipe” that provides constant communication between the gas space in the tank and the atmosphere surrounding outside the tank. In the case that the tank stores a highly volatile liquid therein, a so-called “vent pipe with atmospheric valve” is employed, equipped with a valve operated responsive to a pressure difference between the gas space and ambient atmosphere (See, Hazardous Materials Administration Study Group edited, “Quick reference of hazardous materials facility standards (2)”, Tokyo Institute of Informatics, Feb. 1, 2013, p. 261-263). Such atmospheric valves include a check valve that allows the flow of air from outside of the tank into the gas space and that stops the flow of the gas from the gas space to outside of the tank, for example, a vacuum relief valve.
Such tanks for liquid are typically made of metal. When the gas space in the tank is constantly communication with the atmosphere through a non-valve vent pipe, air containing oxygen enters the gas space through the vent pipe, to be able to form metal corrosion on the inner surface of the tank. In the case in which flammable liquid is stored in the tank, the presence of oxygen exceeding a predetermined concentration might cause the gas in the tank to catch fire.
It is desirable to reduce oxygen concentration as much as possible in the gas space in order to prevent such metal corrosion on the inner surface of the tank or catching fire of flammable liquid stored in the tank. There is an example to prevent the gas in the tank from catching fire by supplying an inert gas (for example, carbon dioxide) having a specific gravity larger than that of air into the tank and forming a layer of an inert gas vertically above the liquid level of the flammable liquid.
However, in the example, the space vertically above the layer of inert gas (shield layer) in the gas space in the tank is in communication with the atmosphere outside the tank, and filled with air containing oxygen. It is difficult to prevent metal corrosion in the portion of the tank inner surface vertically above the layer of inert gas. A volatile gas having a smaller specific gravity than that of air may fill a portion of the gas space vertically above the layer of the inert gas in the tank. The volatile gas might react with the oxygen entered the gas space to catch fire.
It is difficult to completely prevent metal corrosion and catching fire in the tank by merely supplying an inert gas to the gas space in the liquid tank. There is a demand for bringing the oxygen concentration in the gas space as close to zero as possible. Further, there is a demand for maintaining the pressure in the gas space to the desired range close to ambient atmospheric pressure, in order to facilitate pumping of the liquid stored in the tank.